1. Field of the Invention
The invention relates to retinal scanning display devices configured to allow a light beam to be projected directly onto a retina of a viewer, and allow the projected light beam to be scanned onto the retina, to thereby display an image onto the retina. The invention relates more particularly to techniques of expanding an exit pupil of such retinal scanning display devices.
2. Description of the Related Art
There are known retinal scanning display devices which are configured to allow a light beam to be projected directly onto a retina of a viewer, and allow the projected light beam to be scanned onto the retina, to thereby display an image onto the retina.
Such retinal scanning display devices are, in general, configured to include: (a) a light source emitting a light beam; (b) a scanner scanning onto a retina the light beam emitted from the light source; and (c) a light exit at which the light beam scanned by the scanner exits the retinal scanning display devices.
For enabling a viewer to continue viewing a displayed image in a desirable manner through such a retinal scanning display device, there is a need to maintain alignment between a viewer's eye pupil and an exit pupil defined by the retinal scanning display device.
However, it is typical that a viewer's eye moves more or less while viewing a displayed image, inviting movement of a viewer's eye pupil. For this reason, when the exit pupil of the retinal scanning display device is smaller in size than the viewer's eye pupil, a viewer may possibly lose alignment with the exit pupil.
In other words, if the exit pupil of a retinal scanning display device is smaller than a viewer's eye pupil, light exiting the retinal scanning display device may possibly fail to enter the viewer's eye pupil adequately, resulting in an incapability of desirably focusing a display image onto the viewer's retina.
A conventional technique pertaining to a retinal scanning display device having a light source and a light exit is disclosed in U.S. Pat. No. 5,701,132 which is incorporated herein by reference in its entirety.
By this conventional technique, there is positioned between the light source and the light exit a pupil expanding element (e.g., a diffractive element, a diffraction grating, a diffuser) for expanding an exit pupil of the retinal scanning display device, at the location where an intermediate image plane is to occur, in an optical path between the light source and the light exit.